The present invention is directed to a method of extruding thermoplastic elastomer foam using water as a blowing agent. More particularly, the present invention is directed to a method for extruding thermoplastic elastomer foam in which the water and the thermoplastic elastomer are introduced to an extruder simultaneously.
It is known to use water as a mechanical blowing agent in the extrusion of thermoplastic elastomer foam, and particularly the extrusion of such structures. Water is a desirable blowing agent, at least in part because it is non-toxic. Known methods of water blowing thermoplastic elastomers typically introduce the water to the thermoplastic elastomer after the thermoplastic elastomer has melted. Experience with extruding thermoplastic elastomer foam using water as a blowing agent has shown that certain extrusion profiles can be extruded at a rate of 40 to 80 feet per minute.
It is known how to produce low density foams from thermoplastic elastomers using water as a blowing agent which have certain compression or deflection rates, and compression set values, and low water absorption characteristics. However, simpler methods, not requiring the post-melt introduction of water, are sought.
It is desired to have a process for forming thermoplastic elastomer foam with water as a blowing agent suitable for use at increased extrusion speeds.
It is also desired to have a process for foaming thermoplastic elastomer foam which provides better control of the cell structure and the skin characteristics.
It is also desired to have a process for forming thermoplastic elastomer foam in which the water is introduced to the extruder simultaneous with the thermoplastic elastomer.
According to one aspect of the invention, a method for extruding foam of a thermoplastic elastomer, which is a blend of olefin rubber and polyolefin resin, includes the steps of mixing the thermoplastic elastomer with water, introducing the mix to an extruder, melting and compressing the thermoplastic elastomer and water and extruding the resultant mix as foam. In more detail, a first quantity of thermoplastic elastomer in pellet form is mixed with a second quantity of water. The thermoplastic elastomer is allowed to soak for a predetermined period of time after mixing. After soaking, the mixed water and thermoplastic is introduced to an extruder. The thermoplastic elastomer is melted and mixed with the water to a uniform mix of thermoplastic elastomer and water. The mix is extruded through a die, wherein the water expands in a vapor form to create foam cells with the cells having walls of the thermoplastic elastomer.
According to another aspect of the invention, a method for extruding foam of a thermoplastic elastomer, which is a blend of olefin rubber and polyolefin resin, includes the steps of exposing the thermoplastic elastomer to steam, introducing the thermoplastic elastomer having the retained water to an extruder, melting and compressing the thermoplastic elastomer mixing it with the retained water and extruding the resultant mix as foam. As the mix is extruded through the die, the water expands in a vapor form to create foam cells with the cells having walls of the thermoplastic elastomer.
The inventive process enables the use of water as a blowing agent at increased extrusion speeds.
The inventive process enables better control of the cell structure and the skin characteristics.
The inventive process also enables the forming of thermoplastic elastomer foam in which water is introduced as a blowing agent to the extruder simultaneous with the introduction of the thermoplastic elastomer to the extruder.